I'll add that a widely fluctuating water-level allows American lotus to aggressively spread anywhere within the lake that sunlight adequately penetrates the water to sustain newly germinating seeds - which are often very abundant.
Also, topically applied liquid 2,4-D works quite well on lotus, providing that a very good surfactant is added (lotus pads shed water better than a duck's feathers).
I'm currently coordinating aerial applications against several extensive lotus infestations for the sole purpose of interupting the plant's seed production cycle. At least two additional treatments will be required next season for the same purpose - which is the only way to get ahead of this plant specie once it has reached "infestation level".
Depending on your lake's depth-profile, especially around its perimeter, a few patches of lotus are great (both aethetically and for fishing). But, if your lake has extensive areas of 4' or less in depth, be careful not to ignore this plants expansive nature.
